The practice of high resolution NMR has been largely facilitated by progress toward higher magnetic fields. Stable high field instruments are commonly characterized by superconducting magnets. Such a magnet is housed in a cryostat carefully designed to achieve the greatest thermal isolation for a central (liquid helium) cryogen reservoir from ambient temperature. Thermal efficiency of modern cryostats is such that 40 liters of liquid helium may require of the order of 90 days to completely boil away. The cryogen reservoir is necessarily slightly above atmospheric pressure due to the boiling cryogen. Communicating passages from the cryogen reservoir to ambient environment vent the boiling cryogen to atmosphere. Great care must be taken in the design of the venting arrangements to assure that these conduits do not become clogged from condensates.
Notwithstanding the inherent stability of superconducting magnets, other essential elements of the apparatus affect stability and reproducibility. The magnetic field in a typical commercial instrument is capable of exhibiting uniformity to 8th order in the axial coordinate Z, through a combination of both superconducting and room temperature shim coils whereby undesired magnetic field gradients are compensated.
In the present invention, certain environmental instability in NMR apparatus has been remedied. It has been found that high resolution NMR apparatus employing cryogenically-cooled magnets exhibits a drift in line-shape response akin to what would be expected for an unstable gradient power supply. Such instabilities in a gradient power supply(ies) are found to be too small to account for the observed effects. Instead, such observed effects correlate with ambient pressure dependence. In a preferred embodiment, the fill and vent conduits leading to the central cryogen reservoir of the magnet cryostat are adapted to communicate with atmospheric pressure through a constant pressure valve whereby the pressure in the cryogen reservoir is maintained at a selected value independent of barometric variation.